The Synchronous Optical Network (SONET) is a group of standards that define a hierarchical set of transmission rates and transmission formats for carrying high-speed, time-domain-multiplexed (TDM) digital signals. The lowest-rate link in the SONET hierarchy is the OC-1 level, which is capable of carrying 8000 STS-1 frames per second, at a line rate of 51.840 Mbps. A STS-1 frame contains 810 bytes of data, which are conventionally organized as a block of nine rows by 90 columns. The first three columns hold transport overhead (TOH), while the remaining 87 columns carry the information payload, referred to as the synchronous payload envelope (SPE). Multiple STS-1 frames can be multiplexed (together with STS-Mc frames) into STS-N frames, for transmission on OC-N links at rates that are multiples of the basic 51.840 Mbps STS-1 rate.
While the SONET standards have been adopted in North America, a parallel set of standards, known as the Synchronous Digital Hierarchy (SDH), has been promulgated by the International Telecommunication Union (ITU), and is widely used in Europe. From the point of view of the present invention, these alternative standards are functionally interchangeable. In SDH, the information structure used for data transport is the synchronous transport module (STM), which is similar to the SONET STS frame, although the basic STM-1 rate is 155.520 Mbps. The information payload carried by STM frames is referred to as a virtual container (VC-n). The VC-3 container, with a payload rate of 48.384 Mbps, is compatible with the STS-1 SONET rate, while the VC-4 container at 149.760 Mbps is compatible with the STS-3c SONET rate. SDH conventions and data rates are described in detail in ITU-T Recommendation G.707/Y.1322 (October, 2000), entitled “Network Node Interface for the Synchronous Digital Hierarch (SDH).”
For transporting payloads that do not fit efficiently into the standard set of virtual containers, ITU-T Recommendation G.707/Y.1322 (Chapter 11) defines VC concatenation, which allows multiple virtual containers to be combined to provide a desired bandwidth. Two methods for concatenation are defined: contiguous and virtual concatenation. Both methods provide concatenated bandwidth of X times Container-N at the SDH path termination. Contiguous concatenation maintains the contiguous bandwidth through out the whole transport and is limited to X=4, 16 and 64. Virtual concatenation breaks the contiguous bandwidth into individual VCs, transports the individual VCs and recombines these VCs to a contiguous bandwidth at the end point of the transmission. Virtual concatenation requires concatenation functionality only at the path termination equipment, while contiguous concatenation requires concatenation functionality at each network element. The concatenated container, made up of X VC-N containers, is referred to as VC-N-Xc (contiguous concatenation) or VC-N-Xv (virtual concatenation). Virtual-concatenated containers are particularly useful for carrying packet data over SDH or SONET networks, wherein the bandwidth required by the packet traffic is often greater or less than the applicable standard container size.
Changing networks needs and conditions sometimes make it necessary to change the size of the concatenated containers that are in use on a given SONET/SDH link. For this purpose, ITU-T Recommendation G.7042/Y.1305 (November, 2001), which is incorporated herein by reference, defines the “Link Capacity Adjustment Scheme (LCAS) for Virtual Concatenated Signals.” LCAS can be used to increase or decrease the capacity of a container that is transported in a SDH network using virtual concatenation. To change the container capacity on a virtual-concatenated link between a transmitter and a receiver, the transmitter sends control packets to the receiver, indicating the desired change and enabling the receiver to make the change in synchronization with the transmitter. The LCAS mechanism is “hitless,” i.e., it is capable of adjusting the capacity of a link without interrupting or causing errors in the data traffic flowing over the link.